"But you're so tiny. What if you get squished or something. That wouldn't be responsible of me to just let you walk off."

He's being rather lighthearted about what he's saying, just giving another excuse. He doesn't actually think Toushirou will get squished, he's just already made up his mind about going.

"So, where to? Are you going to the grocery store? Or maybe the pet store? I bet you're a dog person. Or maybe more of a cat person since you give off that Leave me alone feeling. A dog can easily be as big as you anyway, then you would really get squished." He laughs a little as they continue walking.